In general, thin film transistors (TFTs) are used as circuits for independently driving respective pixels in liquid crystal displays (LCDs), organic electroluminescent (EL) displays and the like. Such a thin film transistor is formed together with gate lines and data lines on a lower substrate of a display device. That is, the thin film transistor includes a gate electrode which is a part of the gate line, an active layer used as a channel, a source electrode and a drain electrode which are parts of the data line, a gate insulator and the like.
The active layer of the thin film transistor is formed using amorphous silicon or crystalline silicon. However, a thin film transistor substrate using silicon has a drawback of unsuitability for use in flexible display devices due to great weight and inflexibility because it should use a glass substrate. To solve this drawback, recently, a great deal of research has been made on metal oxides.
In addition, research is underway on thin films of zinc oxide (ZnO) as metal oxide used for the active layer of thin film transistors. ZnO thin films are characterized in that crystals are readily grown even at low temperatures and are known as materials for efficiently securing high charge concentration and mobility. However, ZnO thin films are stable upon exposure to air, thus disadvantageously causing deterioration of stability of thin film transistors. In addition, excess carriers generated by oxygen defects may cause problems of increase in off current and variation in threshold voltage.
To improve qualities of ZnO thin films, indium-gallium-zinc oxide (hereinafter, referred to as “IGZO”) thin films in which ZnO thin films are doped with indium (In) and gallium (Ga) have been suggested.
However, when IGZO thin films are deposited using sputters, since deposition is performed using a single target of indium-gallium-zinc with a constant composition, control of a source composition is disadvantageously difficult. In addition, in this case, device properties such as threshold voltage and mobility are disadvantageously deteriorated.